Replacing recursion within for loop with Stream

Question

I am having trouble to replace a for loop which contains recursive call with Stream. The code is about generating proper divisors of a given number when the prime factors of that number is known. The algorithm is taken from here, where it is depicted in an image. This is a part my code for demonstration purpose and it is runnable:

public class Demo {

    private static void generateDivisorsTraditional(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
        for (int i = start; i < primeFactors.elementSet().size(); i++) {
            long prime = Iterables.get(primeFactors.elementSet(), i);
            int count = primeFactors.count(prime);
            ++start;

            for (int c = 0; c <= count; c++) {
                long factor = ArithmeticUtils.pow(prime, c);
                divisors.add(lastFactor * factor);
                generateDivisorsTraditional(start, lastFactor * factor, primeFactors, divisors);
            }
        }
    }

    private static void generateDivisorsStream(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
        IntStream.range(start, primeFactors.elementSet().size())
                .forEach((int i) -> {
                    long prime = Iterables.get(primeFactors.elementSet(), i);
                    int count = primeFactors.count(prime);
                    final int begin = start + 1;

                    IntStream.range(0, count + 1)
                            .forEach((int c) -> {
                                long factor = ArithmeticUtils.pow(prime, c);
                                divisors.add(lastFactor * factor);
                                generateDivisorsStream(begin, lastFactor * factor, primeFactors, divisors);
                            });
                });
    }

    private static void testTraditional(Multiset<Long> primeFactors) {
        Set<Long> divisors = new TreeSet<>();
        generateDivisorsTraditional(0, 1, primeFactors, divisors);
        System.out.println("Traditional=> " + divisors);
    }

    private static void testStream(Multiset<Long> primeFactors) {
        Set<Long> divisors = new TreeSet<>();
        generateDivisorsStream(0, 1, primeFactors, divisors);
        System.out.println("Stream=> " + divisors);
    }

    private static void testStream1(Multiset<Long> primeFactors) {
        Set<Long> divisors = new TreeSet<>();
        new Inner().generateDivisorsStream(1, primeFactors, divisors);
        System.out.println("Stream1=> " + divisors);
    }

    public static void main(String[] args) {
        System.out.println("Test number: 10");
        Multiset<Long> primeFactors = TreeMultiset.create();
        primeFactors.add(2L);
        primeFactors.add(5L);

        testTraditional(primeFactors);
        testStream(primeFactors);
        testStream1(primeFactors);

        System.out.println();

        System.out.println("Test number: 90");
        primeFactors = TreeMultiset.create();
        primeFactors.add(2L);
        primeFactors.add(3L);
        primeFactors.add(3L);
        primeFactors.add(5L);

        testTraditional(primeFactors);
        testStream(primeFactors);
        testStream1(primeFactors);
    }

    private static class Inner {
        private int start = 0;

        private void generateDivisorsStream(long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
            IntStream.range(start, primeFactors.elementSet().size())
                    .forEach((int i) -> {
                        long prime = Iterables.get(primeFactors.elementSet(), i);
                        int count = primeFactors.count(prime);
                        ++start;

                        IntStream.range(0, count + 1)
                                .forEach((int c) -> {
                                    long factor = ArithmeticUtils.pow(prime, c);
                                    divisors.add(lastFactor * factor);
                                    generateDivisorsStream(lastFactor * factor, primeFactors, divisors);
                                });
                    });
        }
    }
}

The output it is generating is:

Test number: 10
Traditional=> [1, 2, 5, 10]
Stream=> [1, 2, 5, 10, 25]
Stream1=> [1, 2, 5]

Test number: 90
Traditional=> [1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90]
Stream=> [1, 2, 3, 5, 6, 9, 10, 15, 18, 25, 27, 30, 45, 50, 75, 81, 90, 125, 135, 225, 405]
Stream1=> [1, 2, 3, 5, 9]

As the name suggests the method generateDivisorsTraditional uses traditional for loop and within which I have a recursive call to the same method. The method generateDivisorsStream uses IntStream.range() to mimic the for loop.

I am suspecting the instructions ++start; and final int begin = start + 1; of generateDivisorsTraditional and generateDivisorsStream respectively are making some differences. I also have tried to use final int begin = start + 1; instead of ++start; in generateDivisorsTraditional and have found that it has started generating wrong result. I have also another variant in the inner class Inner which is also producing wrong output.

I am wondering why this is not behaving the way it is suppose to behave? What is the mistake I have made?

Answer 1

I think there are some mistakes:

private static void generateDivisorsTraditional(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
    for (int i = start; i < primeFactors.elementSet().size(); i++) {
        long prime = Iterables.get(primeFactors.elementSet(), i);
        int count = primeFactors.count(prime);
        // ++start; remove it

        for (int c = 0; c <= count; c++) {
            long factor = ArithmeticUtils.pow(prime, c);
            divisors.add(lastFactor * factor);
            generateDivisorsTraditional(i+1, lastFactor * factor, primeFactors, divisors); // replaced start -> i+1
        }
    }
}

private static void generateDivisorsStream(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
    IntStream.range(start, primeFactors.elementSet().size())
            .forEach((int i) -> {
                long prime = Iterables.get(primeFactors.elementSet(), i);
                int count = primeFactors.count(prime);
                IntStream.range(0, count + 1)
                        .forEach((int c) -> {
                            long factor = ArithmeticUtils.pow(prime, c);
                            divisors.add(lastFactor * factor);
                            generateDivisorsStream(i+1, lastFactor * factor, primeFactors, divisors);
                        });
            });
}

public static void main(String[] args) {
    Multiset<Long> m = HashMultiset.create();
    m.add(1L, 1);
    m.add(2L, 1);
    m.add(5L, 1);
    Set<Long> divisors = new HashSet<>();
    generateDivisorsTraditional(1, 1, m, divisors);
    System.out.println("Traditional=> "+divisors);
    divisors = new HashSet<>();
    generateDivisorsStream(1, 1, m, divisors);
    System.out.println("Stream=> "+divisors);
}

It prints:

Traditional=> [1, 2, 5, 10]
Stream=> [1, 2, 5, 10]